Hit a new problem, so i've installed 10.5.2 on my test partition, downloaded the ASU 0.5, and the 10.5.3update dmg. moved the 10.5.3 pkg into the downloads folder like you said, used ASU to download and modify the airport utility update. It creates a new folder, checks, then patches the Airport utility fine, then the 10.5.3 but it returns "No cpuids found !" and "No need to rebuild !". It then puts both packages into the new folder and finishes. Unfortunately i didn't notice that it didn't actually rebuild the os x update the first time - installing killed my system completely. So essentially it's just moving the file .

Heres my log

####################### Software Update Log #######################

2009 January 17

14:28:35 Preparing download ...
14:29:10 /private/var/folders/5X/5X0mgEqUEF4fvUn3ld1Yl++++TI/-Caches-/com.apple.SoftwareUpdate/swcdn.apple.com/content/downloads/31/13/061-4612/MDs7cYvbZWR6hmMqrmSbnsrKHjrx9DLWQD created
14:29:11 Downloading AirPort Utility 5.3.2 ...
14:29:22 AirPort Utility 5.3.2 downloaded.
14:29:23 Moving packages to temporary folder ...
14:29:26 Extracting AirPortUtility_Leopard.pkg ...
14:29:32 AirPortUtility_Leopard.pkg extracted.
14:29:32 Detecting cpuids for AirPortUtility_Leopard.pkg ...
14:29:37 Done
14:29:38 No cpuids found !
14:29:39 No need to rebuild !
14:29:41 Extracting MacOSXUpd10.5.3.pkg ...
14:30:21 MacOSXUpd10.5.3.pkg extracted.
14:30:22 Detecting cpuids for MacOSXUpd10.5.3.pkg ...
14:30:24 Done
14:30:24 No cpuids found !
14:30:25 No need to rebuild !

has anyone else tried it? maybe i should be using ASU 0.3 rather than 0.5? thanks for all your help